Recently, a power converter is required to have a higher power density (power density=output power of a power converter/volume of a power converter). In order to implement the high power density, the power converter is required to be reduced in volume.
A main part of the volume of the power converter is composed of a heat sink and passive components such as a capacitor and inductor, and in order to miniaturize the heat sink, it is necessary to reduce a loss of a power converter and in order to miniaturize the passive component, it is necessary to increase a switching frequency.
The loss of the power converter mostly lies in a loss of a semiconductor element, and recently, a Si super junction MOSFET (hereinafter referred to as the SiC-MOS) and a SiC MOSFET (hereinafter, referred to as the SiC-MOS) are expected as semiconductor elements capable of lowering the loss.
Since each of the SJ-MOS and the SiC-MOS is a unipolar device in which a carrier to carry a current in a semiconductor is only composed of an electron, its switching speed is high as compared with an IGBT which is a bipolar device in which a carrier is composed of an electron and a hole. Since the high-speed switching can reduce a switching loss, a switching frequency can be increased.
FIG. 2 shows a voltage waveform of a drain-source voltage at the time of a turn-off of the SJ-MOS with an inductive load. At the time of the turn-off, the drain-source voltage rises, and when it reaches Vdc, a voltage oscillates.
This oscillation is generated between a parasitic inductance of a circuit and a junction capacitance of the SJ-MOS, and attenuated by a parasitic resistance of the circuit. At this time, energy accumulated in the parasitic inductance of the circuit is consumed by the parasitic resistance.
The parasitic inductance of the circuit is typically in the order of several tens of nH to several hundreds of nH, and the junction capacitance of the semiconductor element is in the order of several hundreds of pF to several nF, so that a frequency of the oscillation is as high as in the order of several tens of MHz, which causes the problem that it becomes a high-frequency noise source.
Oscillation is suppressed by reducing the switching speed. To reduce the switching speed has been attempted by increasing a gate resistance connected to the semiconductor element.
However, when the switching speed is reduced by the above means, a increase in voltage between the drain and the source of the main switching element and a reduction in drain current of the main switching element are delayed, and as a result, the switching loss problematically increases.